a2+b2=?
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Ta có: a+b+c=0
nên a+b=-c
Ta có: \(a^2-b^2-c^2\)
\(=a^2-\left(b^2+c^2\right)\)
\(=a^2-\left[\left(b+c\right)^2-2bc\right]\)
\(=a^2-\left(b+c\right)^2+2bc\)
\(=\left(a-b-c\right)\left(a+b+c\right)+2bc\)
\(=2bc\)
Ta có: \(b^2-c^2-a^2\)
\(=b^2-\left(c^2+a^2\right)\)
\(=b^2-\left[\left(c+a\right)^2-2ca\right]\)
\(=b^2-\left(c+a\right)^2+2ca\)
\(=\left(b-c-a\right)\left(b+c+a\right)+2ca\)
\(=2ac\)
Ta có: \(c^2-a^2-b^2\)
\(=c^2-\left(a^2+b^2\right)\)
\(=c^2-\left[\left(a+b\right)^2-2ab\right]\)
\(=c^2-\left(a+b\right)^2+2ab\)
\(=\left(c-a-b\right)\left(c+a+b\right)+2ab\)
\(=2ab\)
Ta có: \(M=\dfrac{a^2}{a^2-b^2-c^2}+\dfrac{b^2}{b^2-c^2-a^2}+\dfrac{c^2}{c^2-a^2-b^2}\)
\(=\dfrac{a^2}{2bc}+\dfrac{b^2}{2ac}+\dfrac{c^2}{2ab}\)
\(=\dfrac{a^3+b^3+c^3}{2abc}\)
Ta có: \(a^3+b^3+c^3\)
\(=\left(a+b\right)^3+c^3-3ab\left(a+b\right)\)
\(=\left(a+b+c\right)\left(a^2+2ab+b^2-ca-cb+c^2\right)-3ab\left(a+b\right)\)
\(=-3ab\left(a+b\right)\)
Thay \(a^3+b^3+c^3=-3ab\left(a+b\right)\) vào biểu thức \(=\dfrac{a^3+b^3+c^3}{2abc}\), ta được:
\(M=\dfrac{-3ab\left(a+b\right)}{2abc}=\dfrac{-3\left(a+b\right)}{2c}\)
\(=\dfrac{-3\cdot\left(-c\right)}{2c}=\dfrac{3c}{2c}=\dfrac{3}{2}\)
Vậy: \(M=\dfrac{3}{2}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Do a+b+c= 0
<=> a+b= -c
=> (a+b)2= c2
Tương tự: (c+a)2= b2, (c+b)2= a2
Ta có: \(A=\frac{1}{b^2+c^2-a^2}+\frac{1}{c^2+a^2-b^2}+\frac{1}{a^2+b^2-c^2}\)
\(=\frac{1}{b^2+c^2-\left(b+c\right)^2}+\frac{1}{c^2+a^2-\left(c+a\right)^2}+\frac{1}{a^2+b^2-\left(a+b\right)^2}\)
\(=\frac{1}{-2bc}+\frac{1}{-2ca}+\frac{1}{-2ab}\)
\(=\frac{a+b+c}{-2abc}=0\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Dễ vậy mà ko làm đc àk
\(a_1.a_2=b_1.b_2\Rightarrow\frac{a_1}{b_1}=\frac{b_2}{a_2}\)
\(\Rightarrow\frac{a_1}{b_1}+\frac{a_2}{b_2}=\frac{b_2}{a_2}+\frac{a_2}{b_2}\ge2\sqrt{\frac{b_2}{a_2}.\frac{a_2}{b_2}}=2\) (AM - GM)
có a1.a2=b1.b2
=> a1/b1=b2/a2
có \(\frac{a1}{b1}+\frac{a2}{b2}=\frac{b2}{a2}+\frac{a2}{b2}\)
áp dụng bất đẳng thức cosi cho 2 số dương có
\(\frac{b2}{a2}+\frac{a2}{b2}\ge2\sqrt{\frac{b2}{a2}.\left(\frac{a2}{b2}\right)}=2\)(đpcm)